Method for manufacturing coil-embedded dust core and coil-embedded dust core

ABSTRACT

It is an object to provide a method for manufacturing a coil-embedded dust core with a small variation in inductance value with efficiency and the like. A step (a) of charging soft magnetic metal powder including an insulating material, composing a green body  10 , so as to cover a coil  1 , and a step (b) of compacting the soft magnetic metal powder covering the coil  1  in an axial direction of the coil  1  are included, and in the step (b), the soft magnetic metal powder is compacted while an amount of the soft metal powder charged into the part corresponding to the winding section is kept smaller than an amount of the soft magnetic metal powder charged into the other part that are not corresponding to the winding section, with an upper surface or a lower surface of the winding section as a reference. Thereby, a coil-embedded dust core with entirely uniform density can be obtained, and according to the coil-embedded dust core with entirely uniform density, a variation in inductance value is reduced and it becomes possible to obtain a predetermined inductance value with stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coil-embedded dust core, which may beused in inductors having a unitary structure with a magnetic core and inother electronic components. The present invention also relates to amethod for manufacturing the coil-embedded dust core. More particularly,the invention relates to a method for manufacturing coil-embedded dustcore constructed by embedding an air-core coil in a green body, and thelike.

2. Description of the Related Art

In recent years, electric and electronic equipment has become morecompact, and dust cores that are compact (low in height) yet able toaccommodate large current have come to be in demand.

Materials used for dust cores are ferrite powder and ferromagnetic metalpowder, but ferromagnetic metal powder has larger saturation magneticflux density than ferrite powder and its DC bias characteristics may bemaintained even in a strong magnetic field. Consequently, in making adust core that can accommodate large current, using ferromagnetic metalpowder as a material for dust core has become mainstream.

In addition, in order to further the effort to make the core morecompact (lower in height), a coil body in which a coil and compactedmagnetic powder form a unitary structure has been proposed. In thepresent specification, an inductor having such a structure may be calleda “coil-embedded dust core.”

A manufacturing method for a surface-mount type inductor having astructure of a coil-embedded dust core has been proposed in the past.For example, Japanese Patent Laid-Open No. 5-291046 discloses that anexterior electrode is connected to an insulation-coated conduction wire,and these are enclosed in magnetic powder, which is then compressed intoa magnetic body. Japanese Patent Laid-Open No. 11-273980 discloses thata composite material made by mixing flat soft magnetic metal powder andbinder, and a coil are inserted into a die constituted by a die set anda bottom punch at the same time and compression-forming is performed.Japanese Patent No. 2958807 discloses a method for manufacturing aninductor by compressing magnetic powder while orientating the easy axisof magnetization of magnetic powder along the orientation of themagnetic field formed by energizing the coil in order to obtain a largeinductance value. Further, Japanese Patent No. 3108931 discloses themethod for manufacturing an inductor by preparing a first green body anda second green body which are preformed by compression respectively andby performing main compression-forming until an interface between thefirst green body and the second green body is removed with the coilbeing vertically sandwiched with these green bodies. According to themethod described in Japanese Patent No. 3108931, a loading weight ofmagnetic powder constituting the green body can be increased, andtherefore a larger inductance value can be obtained than in theabove-described Japanese Patent Laid-Open No. 5-291046, Japanese PatentLaid-Open No. 11-273980, and Japanese Patent No. 2958807.

However, according to the method described in Japanese Patent No.3108931, three forming operations are required, that is, preforming ofthe first green body, preforming of the second green body, and mainforming performed with the coil being sandwiched by the first green bodyand the second green body. When these forming operations are performedwith one die machine, a die has to be replaced for each formingoperation, which is inefficient. Further, in the case of main forming,compressing pressure is increased so that the interface of the preformedcore is not left, which causes problems of deformation of the coil, aninsulation failure and the like.

As described above, a large inductance value can be obtained with thecoil-embedded dust core of a small size, and while size reduction ofelectric and electronic devices are rapidly advancing, there is a strongdemand for improvement in quality of the coil-embedded dust core. Inconcrete, as the frequency at which the coil-embedded dust core is usedshifted to a higher frequency side, the demand for precision of theinductance values increases. Since impedance increases in proportion toa frequency, the coil-embedded dust core has to be designed so that theinductance value decreases as the frequency, at which the coil-embeddeddust core is used, is shifted to a higher frequency side. Meanwhile, itis necessary to avoid the situation in which part of the magnetic bodyis saturated magnetically and a predetermined inductance value (designvalue) cannot be obtained. Namely, it is required to obtain aninductance value previously specified based on the working frequencywith stability.

Thus, in view of the above-described points, the present invention hasits object to provide a method for efficiently manufacturing acoil-embedded dust core which attains a predetermined inductance value(design value) with a small variation in inductance value, and the like.

SUMMARY OF THE INVENTION

When the inventors made various studies to solve the above-describedproblems, the inventor found out that the position of the coil in thecoil-embedded dust core and the position especially in the compactingdirection have a large influence on inductance, and a variation of theinductance value is reduced by entirely equalizing the density of thegreen body. The inventors also confirm that it is easy and effective toreduce the amount of soft magnetic metal powder charged into the partcorresponding to the winding section of the air-core coil more than theamount of the soft magnetic metal powder charged into the other partwhich is not corresponding to the winding section in order to equalizethe density of the green body in the coil-embedded dust core entirely.Namely, the present invention is a method for manufacturing acoil-embedded dust core constructed by embedding an air-core coil havinga winding section and end sections led out of the winding section in agreen body, and characterized by including a step (a) of charging softmagnetic metal powder including an insulating material, composing thegreen body, so as to cover the air-core coil, and a step (b) ofcompacting the soft magnetic metal powder covering the air-core coil inan axial direction of the air-core coil, and characterized in that inthe step (b), the soft magnetic metal powder is compacted while anamount of the soft metal powder charged into a part corresponding to thewinding section is kept smaller than an amount of the soft magneticmetal powder charged into the other part that are not corresponding tothe winding section, with an upper surface or a lower surface of thewinding section as a reference.

Here, as the other part, the part corresponding to the hollow part ofthe air-core coil is cited. Namely, compacting is performed in the statein which more soft metal powder is charged into the part correspondingto the hollow part of the air-core coil than the part corresponding tothe winding section. It is also preferable to charge a larger amount ofsoft magnetic metal powder into the parts corresponding to the cornerparts of the green body and the surroundings of the end sections led outform the winding section than into the part corresponding to the windingsection. Since the soft magnetic metal powder charged into the partcorresponding to the winding section of the air-core coil (hereinafter,appropriately called “a coil part”) is easily compacted than the softmagnetic metal powder charged into the other part which is notcorresponding to the winding section (hereinafter, appropriately called“a non-coil part”), the density of the coil part inevitably tends to behigher than that of the non-coil part, but by performing compacting inthe state in which more soft magnetic metal powder is charged into thenon-coil part than into the coil part in advance, the coil-embedded dustcore with entirely uniform density can be obtained. According to thecoil-embedded dust core with entirely uniform density, a variation ininductance value is reduced, and it becomes possible to obtain apredetermined inductance value with stability. Since the coil is metal,it is more difficult to compress than soft magnetic metal powder, andthe coil is sometimes damaged if it is forcefully pressurized. However,according to the method proposed by the present invention, thecoil-embedded dust core with entirely uniform density can be obtainedwithout damaging the coil.

In the aforementioned step (b), a compression ratio of the soft magneticmetal powder in a part corresponding to the maximum number of windingsout of the winding section and a compression ratio of the soft magneticmetal powder in the other part can be made equal. When an air-core coilis made by winding, for example, a conductor 2.5 turns in order to formthe terminals at both sides of the component; there exist a three-turnpart and a two-turn part. In this case, the three-turn part becomes thepart corresponding to the maximum number of windings, and thecompression ratio of this part is usually the highest, but according tothe present invention, it becomes possible to equalize the compressionratios of the soft magnetic metal powder of the part corresponding tothe maximum number of windings out of the winding section and the otherpart, for example, the non-coil part such as the part corresponding tothe hollow part of the coil. Here, the compression ratio in thisspecification is the ratio of the thicknesses of the soft magnetic metalpowder before and after compression.

According to the method for manufacturing the coil-embedded dust coreaccording to the present invention, a density of the green body in thevicinity of an upper surface or a lower surface of the partcorresponding to the maximum number of windings out of the windingsection and a density of the green body in the other part can be madeequal.

Further, the present invention provides a method for manufacturing acoil-embedded dust core in which an air-core coil is embedded in a greenbody with use of a die machine comprising a upper die set including anupper die and a top punch ascending and descending inside the upper die,and a lower die set including a lower die and a bottom punch ascendingand descending inside the lower die. In concrete, in a step (a), softmagnetic metal powder including an insulation material, composing thegreen body, is charged into a cavity of the lower die equipped with atubular member, which has a top portion in substantially the same shapeas the plane shape of the air-core coil, in the bottom punch to beascendable and descendable. In the following step (b), the air-core coilis placed concentrically with the tubular member in a state in which itascends to a predetermined position, inside the cavity of the lower diewith the soft magnetic metal powder being charged therein, and in a step(c), the upper die descends to the lower die, and further charging thesoft magnetic metal powder into a cavity of the upper die so as to coverthe air-core coil. In a step (d), the soft magnetic metal powder iscompacted in the axial direction of the air-core coil by relativelylowering the top punch with respect to the bottom punch. Here, theair-core coil can be a coil made by winding a flat conductor, includinga winding section being insulation coated and end sections led out ofthe winding section. With use of the coil with the flat conductor beingwound around, the current capacity per volume can be increased, andfurther reduction in size of the coil-embedded dust core (reduction inheight) is made possible.

Prior to the aforementioned step (a), it is effective to further includea step of controlling a relative position of the lower die, the bottompunch and the tubular member in a compacting direction according tothickness of the winding section of the air-core coil in the axialdirection. This makes it possible to place the air-core coil at thecenter in the axial direction of the green body ultimately.

Further, in the aforementioned step (d), it is desired that the upperdie, the lower die and the tubular member relatively descend to apredetermined position with respect to the bottom punch while a state inwhich the end sections of the air-core coil are held between the upperdie and the lower die is kept, and in synchronism with the movement torelatively lower the top punch with respect to the bottom punch. Thismakes it possible to pressurize the soft magnetic metal powder in thevertical direction without damaging the end sections of the air-corecoil.

Furthermore, the present invention provides a coil-embedded dust coreincluding a green body in a rectangular parallelepiped shape having afront and back surfaces opposed to each other with a predetermineddistance and a side surface formed on perimeters of the front and backsurfaces, and an air-core coil having a winding section and end sectionsled out from the winding section, with at least the winding sectionbeing placed in the green body, and characterized in that densities ofthe green body are equal in a part corresponding to a maximum number ofwindings out of the winding section and in a hollow part of the air-corecoil. According to the coil-embedded dust core according to the presentinvention, a difference in density between the part corresponding to themaximum number of windings out of the winding section and the hollowpart of the air-core coil can be made only 0.3 g/cm³ or less, wherebythe coil-embedded dust core with a small variation in inductance valuecan be ultimately obtained.

It is desirable that the air-core coil is constructed by a rectangularwire. Further, it is effective to use a so-called terminal-integratedair-core coil, in which part of the air-core coil functions as aterminal section. Furthermore, the end sections of the air-core coil canbe exposed to an outside of the green body from a center of side surfaceof the green body with a thickness direction of the green body as thereference. If the connection part is located inside the green body, ajoint failure (including joint failure) easily occurs to the connectionpart during compression, and by making the end sections of the air-corecoil as the terminal section, and exposing the end sections to theoutside of the green body, the connection part can be placed outside.Thus, the coil-embedded dust core can be provided which hardly causesproblems such as joint failure between the coil and the terminal sectionor insulation failure of the coil and terminal section with respect tothe magnetic powder. In this specification, the connection part meansthe portion at which the components are electrically connected to eachother, and the portion at which soldering is made with the externalelectrode such as a land pattern of the surface mounting substrate iscalled a terminal section. In order to expose the end sections of theair-core coil to the outside of the green body from the center of theside surface of the green body with the thickness direction of the greenbody as a reference, it may be suitable to carry out the method formanufacturing the coil-embedded dust core proposed by the presentinvention with use of the coil with the flat conductor being woundaround and with its both end sections being formed on the same plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional top view of a coil-embedded dust core inaccordance with an embodiment;

FIGS. 2A and 2B are sectional side views of a coil-embedded dust core inaccordance with an embodiment;

FIG. 3 is a plan view of a coil to be used in an embodiment;

FIG. 4 is a side view of a coil used in an embodiment;

FIGS. 5A to 5D are schematic views showing cross-sectional shapes beforeand after a flat conductor is wound;

FIG. 6 is a flowchart of a manufacturing process for a coil inaccordance with an embodiment;

FIGS. 7A and 7B are views for illustrating a winding step;

FIG. 8 is a view for illustrating a forming step;

FIGS. 9A and 9B are views for illustrating a press processing step;

FIGS. 10A to 10C are views for illustrating a bending step;

FIG. 11 is a flowchart of a manufacturing process for a coil-embeddeddust core in accordance with an embodiment;

FIG. 12 is a flowchart illustrating each step of a compressing step instep S206 in FIG. 11;

FIGS. 13A to 13C are views explaining the compressing step in step S206in FIG. 11;

FIGS. 14A to 14C are views explaining the compressing step in step S206in FIG. 11;

FIGS. 15A to 15D are views explaining the compressing step in step S206in FIG. 11; and

FIG. 16 is a graph showing inductance values measured in example 1,comparison example 1, and comparison example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in detail below with reference to anembodiment shown in the accompanying drawings.

FIG. 1 is a cross-sectional top view of a coil-embedded dust coreaccording to this embodiment. FIG. 2A and FIG. 2B are sectional sideviews of the coil-embedded dust core, and FIG. 2B shows a simplifiedview of FIG. 2A. FIG. 3 is a plan view of a coil (air-core coil) 1 usedin this embodiment, and FIG. 4 is a side view of the coil 1. As shown inFIGS. 1 to 4, the coil 1 is an air-core coil including a winding section3 in which a flat conductor 2 is wound and laminated and lead-out endsections 4 a and 4 b each of which is extended from the winding section3. A green body 10 covers the coil 1 and its circumference except thelead-out sections 4 a and 4 b of the coil 1. Although a detaileddescription is given later, in this embodiment, the coil 1 is of what iscalled a terminal integrated construction so that the lead-out endsections 4 a and 4 b of the coil 1 function as a terminal section 100.

As described above, the coil-embedded dust core in the presentembodiment is characterized in that the densities of a partcorresponding to the winding section 3 of the coil 1 (coil part) and anon-coil part (a part corresponding to a hollow part of the coil 1, apart corresponding to corner parts of a green body 10, and surroundingsof end sections led out from the winding section 3) are uniform. Here,as shown in FIG. 2A, the part corresponding to the winding section 3 ofthe coil 1 is the part corresponding to an upper surface and a lowersurface of the winding section 3 in the green body 10 with an axialdirection of the coil 1 (thickness direction) as a reference. The partcorresponding to the hollow part of the coil 1 is the hollow part of thecoil 1, and the part which is the hollow part of the coil 1 extended tothe upper surface and the lower surface of the green body 10 in theaxial direction.

One of the characteristics of the coil-embedded dust core in thisembodiment is that the coil 1 is accurately located at a center in theaxial direction of the green body 10. Namely, as shown in FIG. 2B, inthe coil-embedded dust core in this embodiment, a distance H1 from thelead-out end section 4 a (4 b) to the upper surface of the green body 10and a distance H2 from the lead-out end section 4 a (4 b) to the lowersurface of the green body 10 are equal. A distance H3 from the uppersurface of the winding section 3 of the coil 1 to the upper surface ofthe green body 10 and a distance H4 from the lower surface of thewinding section 3 of the coil 1 to the lower surface of the green body10 are equal.

As described in detail in the embodiment below, in the coil-embeddeddust core in this embodiment, the densities of the part corresponding tothe winding section 3 of the coil 1 shown by {circle over (2)} and{circle over (3)} in FIG. 2B and the part corresponding to the hollowpart of the coil 1 shown by {circle over (1)} in FIG. 2B are equal.

First, the green body 10 is described.

The green body 10 is made by adding an insulating material toferromagnetic metal powder, mixing them, and thereafter compressing themaccording to predetermined conditions. Also, it is preferable that afterthe ferromagnetic metal powder, to which the insulating material isadded, is dried, a lubricant is added to the dried magnetic powder andthey are mixed.

As the ferromagnetic metal powder used in the green body 10, singlemetal powder, two or more kinds of metal powder having a differentchemical composition, or alloy powder can be used. The metal powder canbe composed of any transition metal element exhibiting soft magnetism oran alloy consisting of a transition metal element and other metalelements. As a concrete example of soft magnetic metal, an alloycomposed mainly of one or more kinds of Fe, Co and Ni can be cited. Forexample, Permalloy (Fe—Ni system alloy, Fe—Ni—Mo system alloy), Sendust(Fe—Si—Al system alloy), Fe—Si system alloy, Fe—Co system alloy, Fe—Psystem alloy, and the like are preferable. Among these, Permalloy issuitable because of its high magnetic permeability and excellentworkability.

When an Fe—Ni system alloy (Permalloy) is selected as ferromagneticmetal powder used in the green body 10, the chemical composition shouldbe 15 to 60 wt % of Fe and 40 to 85 wt % of Ni. Also, an Fe—Ni—Mo systemalloy (Permalloy) is selected as ferromagnetic metal powder used in thegreen body 10, the chemical composition should be 15 to 30 wt % of Fe,70 to 85 wt % of Ni, and 1 to 5 wt % of Mo.

The shape of particle of ferromagnetic metal powder used in the greenbody 10 is not limited, a powder with spherical or elliptical particlesis preferably used.

The ferromagnetic metal powder may be obtained by the gas atomizingmethod, water atomizing method, rotary disk method, etc.

By adding the insulating material, the ferromagnetic metal powder isinsulation-coated. The insulating material is properly selecteddepending on the properties of the magnetic core required, and some ofthe materials that may be used as an insulating material are variousorganic polymer resins, silicone resin, phenolic resin, epoxy resin, andwater glass; moreover, a mixture of one of these resins and inorganicsubstances may also be used.

The amount of the insulating material to be added varies depending onthe properties of the magnetic core required, but approximately 1 to 10wt % may be added. When the amount of the insulating material addedexceeds 10 wt %, permeability falls and the loss tends to be larger. Onthe other hand, when the amount of the insulating material added is lessthan 1 wt %, there is a possibility of insulation failure. A desirableamount of insulating material added is 1.5 to 5 wt %.

The amount of the lubricant to be added may be approximately 0.1 to 1.0wt %, the amount of the lubricant to be added may preferably be 0.2 to0.8 wt %, but the more preferable amount of the lubricant to be addedmay be 0.3 to 0.8 wt %. When the amount of the lubricant added is lessthan 0.1 wt %, removing the die after compressing becomes difficult andcracks on the molded product are more likely to occur. On the otherhand, when the amount of the lubricant added exceeds 1.0 wt %, densityfalls and permeability decreases.

The lubricant may be selected from among, for example, aluminumstearate, barium stearate, magnesium stearate, calcium stearate, zincstearate and strontium stearate. Using aluminum stearate as thelubricant is desirable, due to the fact that its so-called spring backis small.

In addition, a predetermined amount of a cross-linking agent may beadded to the ferromagnetic metal powder. Adding the cross-linking agentdoes not deteriorate the magnetic properties of the green body 10, andinstead increases its strength. The amount of the cross-linking agent tobe added may preferably be 10 to 40 wt % to the insulating material suchas silicone resin. The cross-linking agent may be organic titaniumcompounds.

Next, the construction of the coil 1 is described with reference toFIGS. 3 and 4.

As shown in FIGS. 3 and 4, the coil 1 is formed by having the conductor2 wound 2.5 turns in edgewise winding, and the lead-out end section 4 a,4 b of the conductor 2 has a construction such that the conductor 2 isextended from a body section of the coil 1 by inverse forming. That is,the coil 1 is formed integrally without joint.

The cross section of the conductor 2 that forms the coil 1 is flat. Someof the possible flat cross-sectional shapes are rectangular,trapezoidal, or elliptical, for example. The conductor 2 having arectangular cross section may be formed by a rectangular wire made of aninsulation-coated copper wire. When a rectangular wire is used as theconductor 2, the cross-sectional dimensions may preferably beapproximately 0.1 to 1.0 mm long by 0.5 to 5.0 mm wide.

The insulation coating on the conductor 2 may normally be an enamelcoating, and the enamel coating thickness may preferably be about 3 μm.

When the coil 1 is formed by winding the flat conductor 2, the layers ofwinding that makes up the coil 1 can be brought into very close contactwith each other as shown in FIG. 4. Consequently, the electric capacityper cubic volume can be improved, and a product with a smaller heightcan be provided as compared with the case where a conductor having acircular cross section is used. In addition, the wire occupation ratecan be improved significantly as compared with the case where the coil 1is formed by winding a conductor whose number of turns is equal butwhose cross section is circular. Therefore, the coil 1 made by windingthe flat conductor 2 is favorable in making a coil-embedded dust corefor a large current.

FIG. 5 shows the cross-sectional shapes of the flat conductor 2 beforeand after winding.

When a rectangular wire is used as the flat conductor 2, the thicknessof cross section before winding the conductor 2 is uniform as shown inFIG. 5A. When the conductor 2 is wound from this state, its thickness onthe outer circumference side (on the outside of the winding) of the coil1 becomes smaller than its thickness on the inner circumference side (onthe inside of the winding) as shown in FIG. 5B. Here, as describedabove, the coil 1 is formed by winding the conductor 2 a few turns. Whenthe conductor 2 is wound, the windings eventually come into contact witheach other. However, as shown in FIG. 5B, since the thickness of theconductor 2 on the outer circumference side of the coil 1 is madesmaller than its thickness on the inner circumference side by windingthe conductor 2, an air-core coil can be made by winding the conductor 2while preventing the coating on the conductor 2 from being peeled off ordamaged.

If the coil 1 in which the coating of the conductor 2 has peeled off orsuffered damage, were to be embedded within the green body 10, theinductance value of coil-embedded dust core would lower remarkably.

Also, when press processing is rendered in a state in which the flatconductor 2 is wound into a coil and the thickness of the conductor 2 onthe outer circumference side of the coil 1 is smaller than its thicknesson the inner circumference side as shown in FIG. 5C, the outercircumference side of the coil 1 becomes less prone to damage to theinsulation coating. If press processing is rendered in a state in whichthe thickness of the conductor 2 on the outer circumference side of thecoil 1 and the thickness thereof on the inner circumference side aresubstantially equal as shown in FIG. 5D, the insulation coating on theouter circumference side of the coil 1 is more prone to damage.

Based on the cross-sectional shape of the coil 1 formed after theconductor 2 is wound into a coil, the cross-sectional shape of theconductor 2 may be selected to be trapezoidal or the like whenappropriate.

Next, a method for manufacturing the coil 1 in accordance with thisembodiment will be described with reference to FIGS. 6 to 10.

FIG. 6 is a flowchart showing a process for manufacturing the coil 1 inaccordance with this embodiment. As shown in FIG. 6, in the process formanufacturing the coil 1 in accordance with this embodiment, a windingstep of the conductor 2 (step S101), a forming step (step S102), a pressprocessing (flattening) step (step S103), a sizing process step (stepS104), and a bending step (step S105) are included.

<Winding Step of Conductor 2>

First, in step S101, as shown in FIGS. 7A and 7B, the flat conductor 2is wound to form the winding section 3 and the lead-out end sections 4 aand 4 b of the coil 1. The number of turns of the conductor 2 is setappropriately according to the required inductance value, and it can be1 to 6 turns, preferably 2 to 4 turns. FIG. 7B is a side view of thecoil 1 after being wound 2.5 turns in edgewise winding. It is preferablefrom the viewpoint of decreased number of work processes and improvedwire occupation rate that the layers of winding that makes up the coil 1be brought into very close contact with each other in advance at thestage of winding step in step S101 as shown in FIG. 7B.

<Forming Step>

In the succeeding step S102, forming of the coil 1 is performed. FIG. 8is a plan view showing a state in which the lead-out end sections 4 aand 4 b of the conductor 2 are extended from the winding section 3 ofthe coil 1 by inverse forming. The direction, in which the lead-out endsection 4 a is extended, is preferably a direction different from thedirection in which the lead-out end section 4 b is extended. The reasonfor this is that if the lead-out end sections 4 a and 4 b are extendedin the same direction, it is difficult to form the terminal sections 100on both sides of the coil-embedded dust core; inconvenience is causedwhen the lead-out end sections 4 a and 4 b are subjected to pressprocessing (the details of press processing is described later); and itis difficult to arrange the coil 1 in the center of the green body 10when the coil-embedded dust core is manufactured. Also, as shown in FIG.8, forming is preferably performed so that the lead-out end sections 4 aand 4 b are arranged symmetrically. By doing this, when thecoil-embedded dust core using the coil 1 is used as a surface mountingpart, the extending directions of the lead-out end sections 4 a and 4 b,which function as the terminal section 100, can be made symmetrical.Therefore, when the coil 1 is handled, for example, when the coil 1 isplaced in a die machine, the direction of the coil 1 need not bedistinguished.

<Press Processing (Flattening by Pressing) Step>

After the forming of the coil 1 has been performed in step S102, theprocess proceeds to step S103. In step S103, the lead-out end sections 4a and 4 b are subjected to press processing (flattening by pressing;hereinafter referred to as “flattening”.). This step is accomplished tocause the lead-out end sections 4 a and 4 b of the coil 1 to function asthe terminal section 100. Through this step, the plain surfaces of thelead-out end sections 4 a and 4 b are formed so as to be wider andthinner than the plain surface of the conductor 2.

The press processing in step S103 is preferably performed so that thethickness of the conductor 2 is about 0.1 to 0.3 mm. As described above,the press processing is performed to form the plain surfaces of thelead-out end sections 4 a and 4 b so as to be wider and thinner than theplain surface of the conductor 2. In addition, however, an effect thatthe strength of the lead-out end sections 4 a and 4 b functioning as theterminal section 100 is increased by the press processing can beanticipated.

FIG. 9 shows a state after the lead-out end sections 4 a and 4 b havebeen subjected to press processing. FIG. 9A is a plan view of the coil1, and FIG. 9B is a side view of the coil 1.

As shown in FIG. 9A, when the lead-out end section 4 a, 4 b is subjectedto press processing, the conductor 2 in this section elongates in anisotropic manner. Namely, the shapes of the lead-out end sections 4 aand 4 b cannot be made rectangular by simply pressing the conductor 2.Meanwhile, the shape of the lead-out end sections 4 a and 4 b ispreferably rectangular so as to fit to the land pattern of a substrateon which the coil-embedded dust core using the coil 1 is mounted. Thisis because the land pattern tends to be small following the improvementin surface mounting density, and it is necessary to improve precision ofsize and shape of terminals.

<Sizing Process Step>

After the lead-out end sections 4 a and 4 b have been subjected to pressprocessing in step S103, the process proceeds to step S104. In stepS104, the press processed lead-out end sections 4 a and 4 b aresubjected to sizing process. The sizing may be performed by using acutting die, for example. As described above, since the land pattern ofsubstrate on which the coil-embedded dust core is mounted usually has arectangular shape, the lead-out end sections 4 a and 4 b preferably havea rectangular shape to fit to the land pattern. For instance, when usingthe coil-embedded dust core in a notebook computer, the shape of thelead-out end section 4 a, 4 b may preferably be rectangular withdimensions of approximately 20×30 mm to 50×60 mm.

It is not a necessary requirement in making the lead-out end sections 4a and 4 b function as the terminal sections to make the lead-out endsections 4 a and 4 b rectangular, and if the size of the lead-out endsections 4 a and 4 b after the pressing processing is within the landpattern of the substrate, it is possible to omit the sizing process instep S104 appropriately. Although the rectangular shape of the lead-outend section 4 a, 4 b is not an essential requirement for making thelead-out end section 4 a, 4 b function as the terminal section 100 asdescribed above, It should be noted that the requirement for the shapeand dimensional accuracy of the terminal section 100 is strong nowadaysbecause of small and narrow land pattern caused by the increase insurface mounting density. Therefore, the press processed lead-out endsections 4 a and 4 b are preferably subjected to sizing process. Now,the coil 1 having been subjected to sizing process has a planar shape,for example, as shown in the plan view of FIG. 3.

<Bending Step>

After the lead-out end sections 4 a and 4 b have been subjected tosizing process in step S104, the process proceeds to step S105. In stepS105, the sizing processed lead-out end sections 4 a and 4 b aresubjected to bending process. This bending step is characteristic of thepresent invention. This step is performed to arrange the lead-out endsections 4 a and 4 b functioning as the terminal section 100 on the sameplane.

Next, the details of the bending step are explained with reference toFIG. 10. FIGS. 10A to 10C are side views of the coil 1.

FIG. 10A is a side view showing a state in which the lead-out endsections 4 a and 4 b are arranged on the same plane with an intermediatelayer of the winding section 3 being a reference plane. As shown in FIG.10A, when the intermediate layer of the winding section 3 is made areference plane, the lead-out end sections 4 a and 4 b are bent at anangle by the substantially same amount, and bent sections 4 c are formedbetween the lead-out end section 4 a and the winding section 3 andbetween the lead-out end section 4 b and the winding section 3. When thelead-out end sections 4 a and 4 b are arranged on the same plane withthe intermediate layer of the winding section 3 being a reference planein this manner, in the above-described sizing process step (step S104),the lengths of the lead-out end sections 4 a and 4 b are madeapproximately equal, that is, as shown in FIGS. 9A and 9B, a length L1from the centerline of the winding section 3 of the coil 1 to the tipend of the lead-out end section 4 a is caused to coincide with a lengthL2 from the centerline of the winding section 3 of the coil 1 to the tipend of the lead-out end sections 4 b. Thereby, when the bent sections 4c are formed between the lead-out end section 4 a and the windingsection 3 and between the lead-out end section 4 b and the windingsection 3, a length L3 of the lead-out end section 4 a can be caused tosubstantially coincide with a length L4 of the lead-out end sections 4b.

FIG. 10B is a side view showing a state in which the lead-out endsections 4 a and 4 b are formed on the same plane with the uppermostlayer of the winding section 3 being a reference plane, that is, theyare formed so that either one of the front and back surfaces of thelead-out end section 4 a and either one of the front and back surfacesof the lead-out end section 4 b are formed so as to be on the sameplane. As shown in FIG. 10B, when the uppermost layer of the windingsection 3 is used as a reference plane, only one lead-out end section 4b is bent at an angle, by which the bent section 4 c is formed betweenthe lead-out end section 4 b and the winding section 3. Also, when thelead-out end sections 4 a and 4 b are arranged on the same plane withthe lowermost layer of the winding 3 being a reference plane, as shownin FIG. 10C, only one lead-out end section 4 a may be bent at an angleto form the bent section 4 c between the lead-out end section 4 a andthe winding section 3.

When the lead-out end sections 4 a and 4 b are arranged on the sameplane with the uppermost layer of the winding 3 being a reference planeas shown in FIG. 10B, in the above-described sizing process step (stepS104), the length of the lead-out end section 4 b is made longer thanthe length of the lead-out end section 4 a. That is, the above-describedprocess of step S101 through step S104 is performed so that the lengthL2 from the centerline of the winding section 3 of the coil 1 to the tipend of the lead-out end section 4 b is longer than the length L1 fromthe centerline of the winding section 3 of the coil 1 to the tip end ofthe lead-out end section 4 a. The same is true for the case where thelead-out end sections 4 a and 4 b are arranged on the same plane withthe lowermost layer of the winding 3 being a reference plane.

When the bent section 4 c is formed by bending the lead-out end section4 a, 4 b, a portion subjected to flattening may be bent, or a portionnot subjected to flattening may be bent. Since the thickness of thelead-out end section 4 a, 4 b is 0.1 to 1.0 mm before press processingand 0.1 to 0.3 mm after press processing, the lead-out end sections 4 aand 4 b can be bent easily.

The bending step (step S105), which is the step characteristic of thepresent invention, has been described above with reference to FIG. 10.This step is essential in arranging the lead-out end sections 4 a and 4b on the same plane. That is, in a state in which the bent section 4 cis not formed in both portions between the lead-out end section 4 a andthe winding section 3 and between the lead-out end section 4 b and thewinding section 3 as shown in FIGS. 7B and 9B, the lead-out end sections4 a and 4 b cannot be arranged on the same plane.

In the above-described embodiment, an example in which the bending step(step S105) is performed after the press processing step (step S103) andthe sizing process step (step S104) has been explained. However, thepress processing step (step S103) and the sizing process step (stepS104) may be performed after the bending step (step S105) has beenperformed. Also, the bending step (step S105) may be performed betweenthe press processing step (step S103) and the sizing process step (stepS104).

Although described later in detail, when the coil 1 in which thelead-out end sections 4 a and 4 b are arranged on the same plane asshown in FIGS. 10A to 10C is used, an effect that a desired inductancevalue can be obtained and variations in inductance value can be reduced,is achieved. It is a matter of course that the reference plane is notlimited to ones indicated by a solid line in FIGS. 10A to 10C, and areference plane indicated by an imaginary line in FIG. 10C can be used.In this case, the bent section 4 c may be formed so that a distance Hlfrom the predetermined reference plane to the lead-out end section 4 a(either one of the front and back surfaces thereof) is approximatelyequal to a distance H2 from the predetermined reference plane to thelead-out end section 4 b (either one of the front and back surfacesthereof).

Although the method in which the coil 1 is manufactured by performingthe steps of step S101 through step S105 has been described above, thepress processing step (step S103) and the sizing process step (stepS104) can be performed substantially at the same time. This case wherethese two steps are performed substantially at the same time includesboth a case where the sizing process is performed in a state in whichthe lead-out end section 4 a, 4 b functioning as the terminal section100 is subjected to a predetermined pressing force and a case where thesizing process is performed immediately after the lead-out end section 4a, 4 b functioning as the terminal section 100 is subjected to apredetermined pressing force. In order to perform the press processingstep (step S103) and the sizing process step (step S104) substantiallyat the same time, for example, the configuration may be such that acutting die is provided around a punch for press processing, and thecutting die is lowered in the state in which the lead-out end section 4a, 4 b is subjected to the predetermined pressing force or immediatelyafter the lead-out end section 4 a, 4 b is subjected to thepredetermined pressing force to cut the lead-out end section 4 a, 4 binto a predetermined shape.

Further, the press processing step (step S103), the sizing process step(step S104), and the bending step (step S105) can be performedsubstantially at the same time. That is to say, the coil 1 in the stateshown in FIG. 4 can be obtained from the state of coil 1 shown in FIG. 8by one step. In this case, the bent section 4 c may be formed in atleast one portion between the lead-out end section 4 a and the windingsection 3 or between the lead-out end section 4 b and the windingsection 3 by bending a part of the lead-out end section 4 a, 4 b whileapplying the predetermined pressing force to the lead-out end section 4a, 4 b. Immediately after the bent section 4 c has been formed, forexample, a cutting die is lowered to cut the lead-out end section 4 a, 4b into a predetermined shape.

Since, as described above, the coil 1 is formed so that the lead-out endsections 4 a and 4 b function as the terminal section 100, anindependent terminal section need not be provided. That is to say,according to the coil-embedded dust core in accordance with thisembodiment, a connection part between the coil and the terminal sectionis eliminated. The elimination of connection part avoids theconventional problems such as joint failure between the coil and theterminal section and insulation failure of the coil and terminal sectionwith respect to the magnetic powder. Also, since the coil 1 inaccordance with this embodiment is an air-core coil that is made bywinding a flat conductor 2, high inductance value can be provided with asmall number of turns, and downsizing (low in height) of core canfurther be promoted. Further, when the press processing and the sizingprocess are performed substantially at the same time, the number ofprocesses for making the coil 1 can be decreased, so that the workefficiency is improved. Moreover, when the press processing and thesizing process are performed substantially at the same time, the coil 1need not be moved, so that the positioning accuracy at the time ofsizing process becomes higher than before, by which an increase in theworking accuracy of the lead-out end sections 4 a and 4 b that functionas the terminal section 100 can be anticipated. Still further, for thecoil 1 in which the lead-out end sections 4 a and 4 b are arranged onthe same plane, the inductance value varies less, and the performance ishigh.

Next, a method for manufacturing the coil-embedded dust core inaccordance with this embodiment will be described with reference toFIGS. 11 to 15.

FIG. 11 is a flowchart showing a process for manufacturing thecoil-embedded dust core in accordance with the present invention. Thecoil 1 that is formed by winding the flat conductor 2 is manufactured inadvance.

First, a ferromagnetic metal powder and an insulating material areselected according to the required magnetic properties, and they areweighed respectively (step S201). If a cross-linking agent is added, thecross-linking agent is also weighed in step S201.

After weighing out the ferromagnetic metal powder and the insulatingmaterial, they are mixed (step S202). When the cross-liking agent isadded, the ferromagnetic metal powder, the insulating material, and thecross-linking agent are mixed in step S202. The mixing is performed byusing a pressure kneader and preferably at room temperature for 20 to 60minutes. The resultant mixture is dried preferably at a temperature ofabout 100 to 300° C. for 20 to 60 minutes (step S203). Next, the driedmixture is disintegrated to obtain ferromagnetic powder for dust core(step S204).

In the succeeding step S205, a lubricant is added to the ferromagneticpowder for dust core. After the lubricant is added, the powder andlubricant are preferably mixed for 10 to 40 minutes.

After the lubricant is added, a compressing step (step S206) isconducted. The compressing step in step S206 is described below withreference to FIGS. 12 to 15. FIG. 12 is a flowchart illustrating eachstep of a compressing step. FIGS. 13 to 15 show a state in which theferromagnetic powder for dust core, which the lubricant has been addedto and mixed with, is compacted by using a die machine.

First, the die machine preferably used in the compressing process ofthis embodiment will be explained with use of FIG. 13A.

As shown in FIG. 13A, the die machine is constituted by an upper die 5Aand a first lower die 5B, a top punch 6 and a bottom punch 7 and asecond lower die 8. The upper die 5A and the first lower die 5B, and thetop punch 6 and the bottom punch 7 are provided at the positions atwhich they oppose each other, the upper die 5A and the top punch 6ascending and descending in the upper die 5A constitute an upper dieset, and the first lower die 5B and the bottom punch 7 ascending anddescending in the first lower die 5B and a second lower die 8 constitutea lower die set. The bottom punch 7 is divided into a bottom punch mainbody 7 a and a cylindrical divided body (a tubular member) 7 b having atop part in substantially the same shape as the planar shape of the coil1, and the cylindrical divided body 7 b moves ascendably and descendablyin the first lower die 5B. The cylindrical divided body 7 b is includedin the bottom punch 7 to equalize the compacted body densities of thepart corresponding to the winding section 3 of the coil 1 and the otherparts that are not corresponding to the winding section 3 of the coil 1.Namely, this is the idea for charging a smaller amount of ferromagneticpowder for dust core to the part corresponding to the winding section 3of the coil 1 than to the other parts that are not corresponding to thewinding section 3 of the coil 1.

Meanwhile, the reason why a cylindrical divided body corresponding tothe cylindrical divided body 7 b is not provided in the top punch 6 isas follows. Namely, when ferromagnetic powder for the dust core ischarged in the cavity of the die machine in this embodiment, charging byleveling off the powder with use of a feeder box is performed.Correspondingly to this, it is most desired to perform upwardpressurization with use of a punch having a flat surface. Since thebottom punch 7 is divided to take measures to make the compacted bodydensity uniform, it is not necessary to divide the top punch 6. It isnot preferable to divide the top punch 6 from the viewpoint of the cost,and in addition, even if the top punch 6 is divided, compressing cannotbe performed with the procedure that will be described later.

Before a compressing process (step S206) is started, the die machine isin the state shown in FIG. 13A. As will be explained hereinafter, theupper die 5A, the first lower die 5B, the top punch 6, the cylindricaldivided body 7 b and the second lower die 8 changes their positions fromthe state shown in FIG. 13A in each step, but the bottom punch main body7 a is not moved from a predetermined reference plane in any steps.Relative movement of the upper die 5A, the first lower die 5B, the toppunch 6, the cylindrical divided body 7 b and the second lower die 8 inthe compressing process (step S206) will be explained hereinafter, withthe upper surface of the bottom punch main body 7 a as the referenceplane (hereinafter, called “reference plane”).

(Step S301 Primary Charging)

The first lower die 5B, the cylindrical divided body 7 b and the secondlower die 8 ascend to a predetermined position from the state in FIG.13A, namely the reference plane at the same time to form a cavity insidethe first lower die 5B (FIG. 13B). Here, as shown in FIG. 13B, the uppersurfaces of the first lower die 5B and the second lower die 8 arelocated on the same plane, respectively. Since the bottom punch mainbody 7 a does not move, and only the cylindrical divided body 7 bascends, the bottom punch main body 7 a and the cylindrical divided body7 b are located at different levels from each other.

When positioning of the cylindrical divided body 7 b and the first lowerdie 5B is completed, a feeder box F housing a mixed powder 20 (mixtureof the above-described insulation-coated ferromagnetic powder for dustcore with a lubricant) is moved on the first lower die 5B, and charges apredetermined amount of mixed powder 20 into the cavity of the firstlower die 5B. Since the feeder box F performs charging by leveling, theprimary charging amount and the volumetric capacity of the cavity of thefirst lower die 5B become substantially the same. Consequently, it isnecessary to control the positions of the cylindrical divided body 7 band the first lower die 5B previously and accurately based on thethickness of the coil-embedded dust core which is desired to be finallyobtained and the number of windings of the coil 1.

When the mixed powder 20 is charged by leveling into the cavity of thefirst lower die 5B by the feeder box F, the feeder box F is temporarilyretreated.

(Step S302 Rise of the Die 8)

Subsequently, the second lower die 8 accurately ascends to thepredetermined position as shown in FIG. 13C. In concrete, the secondlower die 8 ascends so that an upper surface of a notched part 8 a ofthe second lower die 8 is located on the same plane as the upper surfaceof the first lower die 5B. The second lower die 8 ascends in step S302,but the first lower die 5B and the cylindrical divided body 7 b remainat the same position as that shown in FIG. 13B.

(Step S303 Insertion of the Coil 1)

Next, as shown in FIG. 14A, the coil 1 with the flat conductor 2 beingwounded around is inserted into the first lower die 5B. The coil 1 is anair-core coil previously produced according to the aforementionedprocedure. Engraving (groove) is formed on the upper surface of thefirst lower die 5B to fit to the shapes of the lead-out end sections 4 aand 4 b. In step S303, the coil 1 is placed inside the first lower die5B so that the lead-out end sections 4 a and 4 b are inserted into theengraving. The lead-out end sections 4 a and 4 b are formed on the sameplane as shown in FIG. 10, and therefore when the lead-out end sections4 a and 4 b are inserted to fit in the engraving of the first lower die5B, for example, the coil 1 is horizontally positioned inside the firstlower die 5B without being obliquely positioned. Namely, the coil 1 isultimately positioned horizontally at the center of the green body 10with the horizontal direction as the reference.

(Step S304 Fixing of the Coil 1 and Cavity Formation)

After the coil 1 is inserted into the first lower die 5B in Step S303,the upper die 5A descends to the first lower die 5B as shown in FIG.14B. By the descending of the upper die 5A, the lead-out end sections 4a and 4 b of the coil 1 are sandwiched by the upper die 5A and the firstlower die 5B to be fixed. Consequently, the movement of the coil 1 in alateral direction is controlled. As shown in FIG. 14B, following thedescending of the upper die 5A, a new cavity by the upper die 5A isformed on the upper surface of the coil 1.

(Step S305 Secondary Charging)

When the descending of the upper die 5A is detected by a sensor notshown, the temporarily retreated feeder box F approaches the die machineagain. As shown in FIG. 14C, a predetermined amount of the mixed powder20 is charged inside the cavity that is newly formed in step S304 so asto cover the upper surface of the coil 1. As for the secondary charging,charging by leveling up to the upper surface of the bottom part of theupper die 5A is performed as in the primary charging. Relative positioncontrol of the first lower die 5B, the cylindrical divided body 7 b andthe bottom punch main body 7 a in a compacting direction is previouslyperformed in the aforementioned FIG. 13B so that the coil 1 beaccurately positioned at the center in the axial direction of the greenbody 10.

(Step S306 Lowering of the Top Punch 6)

When the charging is finished in FIG. 14C, the feeder box F is retreatedagain, and at the same time, the top punch 6 descends to the uppersurface of the bottom part of the upper die 5A as shown in FIG. 15A.Namely, in this state, a tip end of the top punch 6 and the uppersurface of the bottom part of the upper die 5A are located on the sameplane.

(Step S307 Synchronism of Material Transfer, Step S308 Pressurization)

At substantially the same time when the top punch 6 descends in stepS306 and the tip end of the top punch 6 and the upper surface of thebottom part of the upper die 5A are located on the same plane (FIG.15A), the upper die 5A, the first lower die 5B, the second lower die 8and the cylindrical divided body 7 b descend in synchronism with the toppunch 6 (FIG. 15B). As a result, the mixed powder 20 sandwiched by thetop punch 6 and the bottom punch 7 is pressurized in the verticaldirection and compressed in the axial direction of the coil 1 (FIG.15C). Here, as described above, the lead-out end sections 4 a and 4 b ofthe coil 1 are sandwiched by the upper die 5A and the first lower die 5Band fixed. Consequently, the upper die 5A and the first lower die 5Bgradually descend according to the compression amount in the axialdirection while holding the lead-out end sections 4 a and 4 b of thecoil 1 so that the lead-out end sections 4 a and 4 b are not broken(FIG. 15B, FIG. 15C). As shown in FIG. 15B, the cylindrical divided body7 b also gradually descends based on the lowering amount of the toppunch 6, namely, the compression amount in the axial direction. Then,the cylindrical divided body 7 b ultimately descends to the referenceplane and stops (FIG. 15C). The condition of pressurization in FIG. 15Band FIG. 15C is desired to be 100 MPa to 600 MPa.

The mixed powder 20 charged in the part corresponding to the windingsection 3 of the coil 1 is compacted more easily than the mixed powder20 charged in the part which is not corresponding to the winding section3, and therefore, if the same amount of the mixed powder 20 is chargedinto each of the part corresponding to the winding section 3 of the coil1 and the parts which are not corresponding to the winding section 3,the density of the green body 10 cannot be ultimately made uniformentirely. Attention is being paid to this, the cylindrical divided body7 b is formed into substantially the same shape as the plane shape ofthe coil 1 and charging is performed so that a smaller amount of themixed powder 20 is charged into the part corresponding to the windingsection 3 than to the parts which are not corresponding to the windingsection 3. As a result, it becomes possible to equalize the density andthe compression ratio of the part corresponding to the winding section 3of the coil 1 and the parts which are not corresponding to the windingsection 3, that are in concrete, the parts corresponding to the hollowpart of the coil 1, the surroundings of the lead-out end sections 4 aand 4 b of the coil 1, and the corner parts of the dust core 10.

Step S306 to step S308 are performed in succession, and the componentsof the die other than the bottom punch main body 7 a, namely, the upperdie 5A, the top punch 6, the first lower die 5B, the second lower die 8and the cylindrical divided body 7 b descend to predetermined positions,respectively in steps S307 and S308.

(Step S309 Drawing)

After compression-forming in FIG. 15C, the upper die 5A and the toppunch 6 ascend as in FIG. 15D, and the first lower die 5B and the secondlower die 8 descend to the reference plane. Then, the compacted body(coil-embedded dust core) is drawn out of the die, whereby one cycle ofcompressing step is finished. On the occasion of drawing in step S309,the first lower die 5B, the second lower die 8, the bottom punch mainbody 7 a and the cylindrical divided body 7 b are positioned on thereference plane. Since the upper die 5A and the top punch 6 also ascendto the original position, in FIG. 15D, the die machine is returned tothe state shown in FIG. 13A. A ring-shaped trace corresponding to theshape of the top part of the cylindrical divided body 7 b remains on thecompacted body which is obtained through the aforementioned compressingstep (step S206).

As a result of undergoing the steps shown in the above steps shown instep S301 to step S309, a small-sized compacted body (coil-embedded dustcore) of about 5 mm to 15 mm long, 5 mm to 15 mm wide, and 2 mm to 7 mmthick can be obtained. According to the manufacturing method of thecoil-embedded dust core of the present invention, preforming is notrequired, and only one compressing forming is sufficient. Accordingly,the present invention is excellent in working efficiency andproductivity. The structure of the die machine is devised, thecylindrical divided body 7 b is formed into substantially the same shapeas the plane shape of the coil 1, charging is performed so that asmaller amount of the mixed powder 20 is charged to the partcorresponding to the winding section 3 than to the parts which are notcorresponding to the winding section 3, and thereafter pressurization isperformed. As a result, the density of the compacted body can be madeuniform.

By accurately controlling the position of the compacting direction withuse of the method for manufacturing the coil-embedded dust core of thepresent invention, the position in the axial direction of the coil 1 canbe accurately positioned at the center of the green body 10 as shown inFIG. 2B. As described above, the position of the coil 1 in thecompacting direction has a large influence on inductance of thecoil-embedded dust core, and accurate positioning of the coil 1 in theaxial direction at the center of the green body 10 can provide a largeinductance value and reduces variations in inductance valuesignificantly. It is considered that the variation of the inductancevalue is reduced as described above because the magnetic path length ofthe coil-embedded dust core and a sectional area can be controlled to bepredetermined values by accurately controlling the axial position of thecoil 1. When the position in the axial direction of the coil 1 isinclined, magnetic saturation easily occurs locally, and a tendency thatthe inductance value decreases can be seen, but according to the methodfor manufacturing the coil-embedded dust core of the present invention,such a problem does not occur and a desired inductance value can beobtained with stability. The operation of the die machine is explainedwith the upper surface of the bottom punch body 7 a as the referenceplane so far, but the reference plane is not limited to what isdescribed above if only the die machine relatively moves similarly.

After the compressing step in step S206 shown in FIGS. 11 and 12, acuring step (heat treatment step) is conducted (step S207). In thecuring step, the compacted body obtained in the compressing step (stepS206) is kept at temperatures of 150 to 300° C. for 15 to 45 minutes. Bydoing this, the resin within the compacted body is hardened.

After the curing step, a rust-proofing step is conducted (step S208).Rust-proofing is done by spray coating epoxy resin, for example, on thecompacted body consisting of the coil 1 and the green body 10. Thethickness of the coat resulting from the spray coating is approximately15 μm. After rust-proofing, the compacted body is preferably subjectedto heat treatment at 120 to 200° C. for 15 to 45 minutes.

As described above, in the coil-embedded dust core in accordance withthis embodiment, a part of the coil 1 is used as the terminal section100. However, the conductor 2 used in the coil-embedded dust core has aninsulation film such as an enamel film formed on the surface thereof tobegin with. According to the observation made by the inventors, a copperoxide film is formed directly underneath the insulation film in thecuring step in step S207. Further, a paint film is formed on theinsulation film through the rust-proofing step (step S208). These filmsformed on the terminal section 100 are removed in a sandblasting step(step S209).

One way to remove the three layers of films formed on the surface of thecoil 1 is to corrode them with chemicals. However, because differentchemicals are required to remove different films, several treatmentsmust be rendered in order to remove the three layers of films. Inaddition, the chemical corrosion method requires heating of thechemicals, which entails a risk of alkaline particles or acidicparticles attaching to the paint film or the insulation film of theterminal section 100 when the chemicals are heated. Such attachmentwould result in progressive corrosion of the paint film or theinsulation film over a long period of time, which is likely to causelowered rust-proofing efficiency or a short circuit between the coil 1layers. To avoid such a risk, there is available a mechanical removingmethod using a tool. However, a tool that may damage the copper part ofthe conductor 2, cannot be used because the thickness of the terminalsection 100 of the coil-embedded dust core in accordance with thisembodiment is about 5 mm or smaller (about 0.1 to 0.3 mm). Consequently,in this embodiment, a method for removing the three layers of films bysandblasting is used.

When the terminal section 100 is to be surface mounting terminalsection, the terminal section 100 is soldered (step S210). Thereafter,it would be convenient to bend the terminal section 100 which has becomewide through flattening process, as necessary when mounting thecoil-embedded dust core on a substrate. In that case, terminal section100 is bent along the side of the green body 10.

The following effects may be obtained from the coil-embedded dust coreaccording to this embodiment.

-   (1) Since the position of the coil 1 in the axial direction is    accurately positioned at the center of the green body 10 and the    density of the compacted body is uniformed entirely, the variations    of the inductance value is significantly reduced, and a    predetermined inductance value can be obtained with stability.-   (2) Because the coil 1 is formed by winding the flat conductor 2,    high inductance can be obtained with a small number of turns. Also,    a compact (low in height) coil-embedded dust core measuring 5 to 15    mm long by 5 to 15 mm wide by 2 to 7 mm thick can be obtained.-   (3) Because the lead-out end section 4 a, 4 b, which is a part of    the coil 1, is used as the terminal section 100, there is no need    for forming a connection part between the coil 1 and the terminal    section. Therefore, problems of joint failure and insulation failure    caused by the connection part can be solved.-   (4) Because the lead-out end sections 4 a and 4 b are formed on the    same plane, positioning can be performed easily and exactly when the    coil 1 is arranged in the die machine. Thereby, the mixed powder 20    can be filled uniformly, so that the inductance value varies less.-   (5) Because the lead-out end section 4 a, 4 b, which is a part of    the coil 1, is used as the terminal section 100, there is no need    for preparing a terminal section separately. Therefore, the number    of parts can be decreased.

EXAMPLE

The coil-embedded dust core in accordance with the present inventionwill be described in detail by using an example.

Example 1

Thirty samples of the coil-embedded dust core having a core size of 12.5mm long×12.5 mm wide×3.5 mm thick were made according to the followingprocedure:

The following were prepared:

-   -   Magnetic powder: Permalloy powder manufactured through atomizing        method (45% Ni—Fe; mean particle size 25 μm)    -   Insulating material: silicone resin (SR2414LV by Toray Dow        Corning Silicone Co., Ltd.)    -   Lubricant: aluminum stearate (SA-1000 by Sakai Chemical        Industry)

Next, 2.4 wt % of the insulating material was added to the magneticpowder, and these were mixed for 30 minutes at room temperature using apressure kneader. Following this, the mixture was exposed to air anddried for 30 minutes at 150° C., thereafter 0.4 wt % of the lubricantwas added to the dried magnetic powder and mixed for 15 minutes in a Vmixer.

Subsequently, compressing was performed according to the procedure inFIG. 13A to FIG. 15D, and 30 compacted bodies are made. The coil 1 isformed by winding the inductor 2 with a rectangular section (0.45 mm×2.5mm) 2.5 turns. Pressure in FIG. 15D is set at 490 MPa. By heat-treatingthe compacted body after pressurized at 200° C. for 15 minutes, siliconeresin as an insulating material is hardened, and the terminal section100 was bent, whereby thirty samples of the coil-embedded dust coreswere made. The inductance values of thirty samples are shown in FIG.16A. “0A” and “20A” in FIG. 16A show a value of a direct current whichis superposed on the inductance measuring AC signal (0.05 V, 100 kHz).

As shown in FIG. 16A, it is understood that the samples according tothis embodiment for which compressing is performed according to theprocedural steps in FIG. 13A to FIG. 15D have a small variation ininductance value. In concrete, in the case with only an alternatecurrent (in the case in which the superposed direct current is 0A), theinductance values were all within the range of 0.60 to 0.64 μH, and thedifference between the minimum value and the maximum value was only 0.04μH. From FIG. 16A, it is understood that in the case in which the directcurrent of 20A is superposed, the same tendency as in the case with onlythe alternate current is also shown. Namely, all the inductance valueswere in the range of 0.53 μH to 0.57 pH, the difference between theminimum value and the maximum value was only 0.04 μH. Consequently,samples of the present invention which were compressed according to theprocedural steps in FIG. 13A to FIG. 15D have a small variation in theinductance value and superior DC bias characteristics.

Next, the density of the center parts of the samples was measured withGamma Densomat made by Creveserge (a density measuring device using γrays). As a result, all of the density of the hollow part {circle over(1)} of the coil 1, the density of the part {circle over (2)}corresponding to the lower surface of the winding section 3 of the coil1, and the density of the part {circle over (3)} corresponding to theupper surface of the winding section 3 of the coil 1 were 6.4 to 6.5g/cm. As for the densities of the part {circle over (2)} correspondingto the lower surface of the winding section 3 of the coil 1, and thepart {circle over (3)} corresponding to the upper surface of the windingsection 3 of the coil 1 those shown in FIG. 2B, the density of the partwith the maximum number of windings, that is, the part with three turnswas measured.

When the position in the compacting direction (thickness direction) ofthe coil 1 was measured from the X-ray projection photograph (made byShimadzu Corporation), it was recognized that the coil 1 was positionedat the center in the axial direction of the green body 10.

Comparison Example 1

Before the coil 1 is inserted in FIG. 14A, the lower core was preformed.Before main pressurization in FIG. 15D, the bottom punch 7 was replacedwith the one having the flat surface, and the main pressurization wasperformed. With the same procedure as in the example 1 except for this,thirty samples of the coil-embedded dust cores were made. On makingthese samples, the pressure in the preforming was set at 150 MPa, andthe main pressurization in FIG. 15D was performed at 490 MPa. Theinductance values of the samples in comparison example 1 are shown inFIG. 16B. The measurement condition of the inductance values is the sameas in the example 1.

When FIG. 16A and FIG. 16B are compared, it is understood that thesamples of which inductance values are shown in FIG. 16B, that is, thesamples with the lower cores being preformed have large variations inthe inductance values. In concrete, in the case with only an alternatecurrent (in the case in which the superposed direct current was 0A), theinductance values of the samples were all in the range from 0.53 to 0.61μH, and the difference between the minimum value and the maximum valuewas 0.08 μH. When the direct current of 20A was superposed, theinductance values of the samples were all in the range of 0.46 to 0.54μH, and the difference between the minimum value and the maximum valuewas 0.08 μH.

Next, as a result of measuring the densities of the center parts of thesamples in Comparison example 1 as in the example 1, the densities werefrom 6.6 to 6.8 g/cm³.

When the position of the coil 1 was measured from the X-ray projectionphotograph (made by Shimadzu Corporation), it was recognized that theposition of the coil 1 in the compacting direction (thickness direction)was deviated upward from the center in the axial direction of the greenbody 10.

From the above result, it was found out that the samples with the lowercores being preformed have little high density at the center parts ascompared with the samples of the example 1 for which compression wasperformed according to the procedural steps in FIG. 13A to FIG. 15D, buthave larger variations in the inductance values and smaller inductancevalues than the samples of the example 1. It is assumed that this isbecause magnetic saturation occurs locally since the position of thecoil 1 is deviated from the center of the green body 10 in the axialdirection.

Comparison Example 2

Thirty samples of the coil-embedded dust cores were made by the sameprocedure as in the example 1 except for the following points.

In the steps in FIG. 13A to FIG. 15D, compression was performed with thecylindrical divided body 7 b being fixed at the reference plane. Namely,the charging amount of the mixed powder 20 was not adjusted for the partcorresponding to the winding section 3 of the coil 1 and the parts whichare not corresponding to the winding section 3 of the coil 1. In FIG.13B and FIG. 13C, the position control of the first lower die 5B and thesecond lower die 8 was not performed. The inductance values of thirtysamples thus made are shown in FIG. 16C.

As shown in FIG. 16B, it is understood that variations in inductancevalues of the samples made in the comparison example 2 are large as thesamples made in the comparison example 1 (see FIG. 16C). In concrete,the inductance values in the case with only the alternate current (inthe case in which the superposed direct current is 0A) were all in therange from 0.55 to 0.63 μH, and the difference between the minimum valueand the maximum value was 0.08 μH. When the direct current of 20A wassuperposed, the inductance values were all within the range from 0.47 to0.56H, and the difference between the minimum value and the maximum valewas 0.09 μH.

Next, the densities of the center parts of the samples were measuredwith Gamma Densomat made by Creveserge (a density measuring device usingγ rays) as in the example 1. As a result, all of the density of the part{circle over (2)} corresponding to the lower surface of the windingsection 3 of the coil 1 shown in FIG. 2B, and the density of the part{circle over (3)} corresponding to the upper surface of the windingsection 3 of the coil 1 were 6.4 to 6.5 g/cm³, while the density of thehollow part {circle over (1)} of the coil 1 was 5.0 to 5.4 g/cm³.Namely, it was found out that in the samples made in the example 1, thedifference in density between the parts {circle over (2)} and {circleover (3)} corresponding to the upper and lower surfaces of the windingsection 3 of the coil 1 and the part corresponding to the hollow part{circle over (1)} of the coil 1 is only 0.1 g/cm³, while in the samplesmade in the comparison example 2, the difference in the density betweenthe parts {circle over (2)} and {circle over (3)} corresponding to theupper and lower surfaces of the winding section 3 of the coil 1 and thepart corresponding to the hollow part {circle over (1)} of the coil 1 islarge and 1.4 g/cm³ or more.

When the position of the coil 1 was measured from the X-ray projectionphotograph (made by Shimadzu Corporation), it was recognized that theposition of the coil 1 in the compacting direction (thickness direction)was deviated upward or downward from the center in the axial directionof the green body 10.

Example 2

Out of the thirty samples made in the example 1, twenty samples werebroken and the densities of the parts corresponding to the windingsections 3 of the coils 1 and the densities of the hollow parts of thecoil 1 shown in FIG. 2A were measured using an Archimedean method withsilicone oil. The result is shown in Table 1. Since the weight of eachpart is small, each part was taken out from twenty samples, and wasmeasured together. The specific gravity of silicone oil is 0.817. TABLE1 In In air silicone Density (g) oil (g) (g/cm³) Density of part 8.5107.441 6.50 corresponding to winding section 3 of coil 1 Density of part7.249 6.327 6.42 corresponding to hollow part of coil 1

As shown in Table 1, the density of the part corresponding to thewinding section 3 of the coil 1 shown in FIG. 2A was 6.50 g/cm³, and thedensity of the part corresponding to the hollow part of the coil 1 was6.42 g/cm³. Namely, the difference between the density of the partcorresponding to the winding section 3 of the coil 1 and the partcorresponding to the hollow part of the coil 1 was only 0.08 g/cm³. Fromthis result, it was confirmed that according to the method which thepresent invention recommends, the coil-embedded dust core with uniformdensity in entirety can be obtained.

While the description above refers to embodiments and examples of thepresent invention, it will be understood that various modifications andchanges may be made without limiting thereto within the range of theclaims.

As explained thus far, according to the present invention, thecoil-embedded dust core which attains a predetermined inductance value(design value) with a small variation in inductance value can beefficiently manufactured. According to the present invention, it is notnecessary to increase compression pressure, and therefore deformation ofthe coil, an insulation failure and the like hardly occur.

1. A method for manufacturing a coil-embedded dust core constructed byembedding an air-core coil having a winding section and end sections ledout of said winding section in a green body, comprising: a step (a) ofcharging soft magnetic metal powder including an insulating material,composing said green body, so as to cover said air-core coil; and a step(b) of compacting said soft magnetic metal powder covering said air-corecoil in an axial direction of said air-core coil, wherein in said step(b), said soft magnetic metal powder is compacted while an amount ofsaid soft metal powder charged into a part corresponding to said windingsection is kept smaller than an amount of said soft magnetic metalpowder charged into the other part that is not corresponding to saidwinding section, with an upper surface or a lower surface of saidwinding section as a reference.
 2. A method for manufacturing thecoil-embedded dust core according to claim 1, wherein said other part isa part corresponding to a hollow part of said air-core coil.
 3. A methodfor manufacturing the coil-embedded dust core according to claim 1,wherein in said step (b), a compression ratio of said soft magneticmetal powder in a part corresponding to the maximum number of windingsout of said winding section and a compression ratio of said softmagnetic metal powder in said other part are equal.
 4. A method formanufacturing the coil-embedded dust core according to claim 1, whereina density of said green body in the vicinity of an upper surface or alower surface of the part corresponding to the maximum number ofwindings out of said winding section and a density of said green body insaid other part are equal.
 5. A method for manufacturing a coil-embeddeddust core in which an air-core coil is embedded in a green body with useof a die machine comprising a upper die set including an upper die and atop punch ascending and descending inside said upper die, and a lowerdie set including a lower die and a bottom punch ascending anddescending inside said lower die, comprising: a step (a) of chargingsoft magnetic metal powder including an insulation material, composingsaid green body, into a cavity of said lower die equipped with a tubularmember, which has a top portion in substantially the same shape as theplane shape of said air-core coil, in said bottom punch to be ascendableand descendable; a step (b) of placing said air-core coil concentricallywith said tubular member in a state in which it ascends to apredetermined position, inside the cavity of said lower die with saidsoft magnetic metal powder being charged therein; a step (c) of loweringsaid upper die to said lower die, and further charging said softmagnetic metal powder into a cavity of said upper die so as to coversaid air-core coil; and a step (d) of compacting said soft magneticmetal powder in an axial direction of said air-core coil by relativelylowering said top punch with respect to said bottom punch.
 6. A methodfor manufacturing the coil-embedded dust core according to claim 5,wherein said air-core coil is a coil made by winding a flat conductor,including a winding section being insulation coated and end sections ledout of said winding section, and prior to said step (a), said methodfurther comprises a step of controlling a relative position of saidlower die, said bottom punch and said tubular member in a compactingdirection according to thickness of said winding section of saidair-core coil in the axial direction.
 7. A method for manufacturing thecoil-embedded dust core according to claim 5, wherein said air-core coilis a coil made by winding a flat conductor, including a winding sectionbeing insulation coated and end sections led out from said windingsection, and in said step (d), said upper die, said lower die and saidtubular member relatively descend to a predetermined position withrespect to said bottom punch while a state in which said end sections ofsaid air-core coil are held between said upper die and said lower die iskept and in synchronism with the movement to relatively lower said toppunch with respect to said bottom punch.
 8. A coil-embedded dust corecomprising: a green body in a rectangular parallelepiped shape having afront and back surfaces opposed to each other with a predeterminedistance and a side surface formed on perimeters of said front and backsurfaces; and an air-core coil having a winding section and end sectionsled out from said winding section, with at least said winding sectionbeing placed in said green body, wherein densities of said green bodyare equal in a part corresponding to a maximum number of windings out ofsaid winding section and a hollow part of said air-core coil.
 9. Acoil-embedded dust core according to claim 8, wherein a difference indensity between the part corresponding to the maximum number of windingsout of said winding section and the hollow part of said air-core coil is0.3 g/cm³ or less.
 10. A coil-embedded dust core according to claim 8,wherein said air-core coil is constructed by a rectangular wire.
 11. Acoil-embedded dust core according to claim 8, wherein part of saidair-core coil functions as a terminal section.
 12. A coil-embedded dustcore according to claim 8, wherein said end sections are exposed to anoutside of said green body from a center of the side surface of saidgreen body with a thickness direction of said green body as thereference.